Phase (waves)
![Illustration of phase shift. The horizontal axis represents an angle (phase) that is increasing with time.](/Images/godic/202502/02/Phase_shift.svg3246.png")
![Left: the real part of a plane wave moving from top to bottom. Right: the same wave after a central section underwent a phase shift, for example, by passing through a glass of different thickness than the other parts. (The illustration on the right ignores the effect of diffraction, which would make the waveform continuous away from material interfaces and would add increasing distortions with distance.).](/Images/godic/202502/02/Phase-shift_illustration3246.png")
Phase in sinusoidal functions or in waves has two different, but closely related, meanings. One is the initial angle of a sinusoidal function at its origin and is sometimes called phase offset or phase difference. Another usage is the fraction of the wave cycle that has elapsed relative to the origin.